>Why is deciding what speech / UGC should be allowed in social media a 'corporate responsibility' such that letting the government or some 'impartial' NGO arbitrate this a bad thing?
Because there is far more freedom to reject a corporation than the government (the NGO would depend upon how it interacts with the government). NGOs and governments are susceptible to bias, and while they may be less susceptible than a private corporation, they have far stronger enforcement of what ever bias they do have (once again assuming the NGOs are somehow working with government, if not then this only applies to government).
Take for example someone determining what is allowed to be printed in a book. Let's go back about two or three decades, so during the timeframe when most of us were alive even if not at all aware of politics, when social norms and even laws enforced views that we would find objectionable today.
Now, consider two possibilities. One, corporations decides what isn't allowed and thus chooses to ban any pro-LGBT content. Two, government decides what isn't allowed and thus chooses to ban any pro-LGBT content. Which is better?
In my mind, the corporate ban is far better because it is much easier for other smaller corporations to be started that will publish pro-LGBT content. As social norms change, corporate enforcement of morals give rise to more freedom.
Perhaps an even better example that is still relevant today instead of a few decades ago: according to the federal government marijuana has no medical benefits.
I'd rather lawmakers not be making any more laws that further restrict my freedoms.